The invention relates to fault detection circuits, and more particularly relates to fault detection circuits for multi-phase loads. In its most immediate sense, the invention relates to fault detection circuits which are used in high-reliability three-phase aerospace applications where the proper performance of critical loads, such as anti-icing or deicing heaters, or three-phase AC motors, must be continuously monitored.
Aircraft conventionally use 400 Hz, three-phase power supplies. Anti-icing and deicing heaters, such as those on engine inlet fairings and leading surfaces of wings, are typically in wye-configuration.
These heaters must function properly and the pilot must be informed if they cease to do so. If such a heater malfunctions, a large chunk of ice can accrete and, e.g., damage the engine when the chunk breaks off. Therefore, airframe manufacturers have long required the provision of fault detection circuitry to automatically alert the pilot if, e.g. a critical heater develops a fault that causes it to operate improperly.
Each phase of wye-connected loads of this type typically contains many circuits in parallel. As the number of such circuits increases, so too does the difficulty of accurately detecting the failure of a single circuit. This is because such a failure may cause only a small change in the resistance of that phase of the load.
As a theoretical matter, it would be possible to tap each of the many circuits in a wye-connected load and to monitor the voltage across each circuit in a fault detection circuit. However, two main reasons make this solution impractical. First, there would be many taps and it would be difficult and costly to route the necessary wiring through, e.g., a heated fairing. Second, the voltages of the various phases of the source can vary during normal operation. Such variation, reflected across a single circuit in a single phase, could easily be interpreted as a malfunction.
It would therefore be advantageous to provide a circuit for monitoring heaters of this type which would be sensitive enough to respond even to small changes in resistance of the circuit, while being unresponsive even to large changes in voltage of the phases of the source.
One object of the invention is to provide a fault detection circuit for determining the existence of a fault in any one of a plurality of centerpoint-connected loads which are supplied by a multi-phase source.
Another object is to provide such a circuit which is highly sensitive to faults in the loads and highly insensitive to changes in the source.
A further object is to provide such a circuit which does not require the use of many conductors which must be routed out of the loads.
Still a further object is, in general, to improve on prior art circuits of this general type.
The invention proceeds from the realization that where there are a plurality of centerpoint-connected loads all supplied by a single multi-phase source, variations in voltage of the source phases, and indeed shorting of one or more phases to ground, will affect the centerpoints of all of the loads in the same way. On the other hand, where a fault develops in one of the loads, the centerpoint of that load will be affected while the centerpoints of the other loads will not.
In accordance with the invention, a reference centerpoint-connected load is connected to the multi-phase source and the reference centerpoint of that reference load is used as a benchmark. The voltages at the centerpoints of all the loads to be monitored are themselves monitored and the largest monitored voltage is identified. This largest voltage is then compared against the voltage of the centerpoint of the reference load.
Where a load fault has developed, the comparison makes the fault manifest even if the fault is relatively small, because a small voltage change in one phase of the load will cause a large shift in the voltage of the centerpoint of the load. Alternatively, where a source fault (e.g. a change in voltage of one of the source phases, a shorting of one or more phases to ground, etc.) takes place, all of the loads (including the reference load) are affected identically and the comparison shows no change as it does with a load fault.